Electrolytic refining and recovery of metal values from leach solutions is well known. The principal activity in this area has been, however, in the better grades of ore. Where the primary metal to be recovered is in too low concentration, e.g., in the case of copper less than about 0.5 percent by weight, it is uneconomical to recover the copper and such ore is frequently passed over as worthless. Too much material must be handled for current prices to justify the effort. The prior art has reported it to be all but impossible to extract the last of the copper electrolytically at a profit. (See U.S. Pat. No. 1,195,616, Column 3, Line 28).
Ordinarily in electroplating by electrolysis, the manufacture of the chlorine by electrolysis, and such other electrochemical reactions it is desirable to have maximum current density for the most rapid exchange of electrons and hence electrochemical reaction. However, the intensity of the current used or the current density is directly proportional to the concentration of, for example, the metal being recovered from the ion containing and conducting medium. As the ion containing and conducting medium is depleted of the metal being plated, for example, the efficiency of the cell decreases rapidly. It has, therefore, been common practice to use a given cell at maximum current density until a predetermined concentration of the desired ion has been reached or efficiency has materially decreased, and then transfer the partially spent ion containing and conducting medium to another cell where a lower current density is being applied. Alternatively, the current density in a cell such as that first described may be manually adjusted as the desired metal ion is depleted from the ion containing and conducting medium so as to correspond more nearly to the strength of that medium. This is impractical. Thus, it has been common practice to utilize a series of individual cells using decreasing current densities in succeeding cells until the ion being recovered is finally depleted at the end of the series of cells.
The present invention solves these problems by providing an apparatus in which the current drain automatically decreases in an axial direction as the concentration of the desired ion in the ion containing and conducting medium also decreases. In like manner where an electrochemical reaction is being carried out as the concentration of a desired product increases, and the demand for conversion of the raw material decreases, also the current drain will automatically decrease. This improves the efficiency of the overall cell.
Also, the axial flow type cell of the present invention enables efficient recovery of low concentrations of desired ions from the ion containing and conducting medium which concentrations were prior to this time thought to be uneconomically recoverable. The improved cells of the present invention have the advantage in that a single cell utilizing an axial flow principle for the ion containing and conducting medium replaces the multiple cell procedure previously described, as well as those where the electrolyte followed a tortuous path between and around a stack of electrodes.